1. Field of the Invention
This invention relates in general to an improvement of a charging roller used in an image forming device such as a copy machine, a printer, a facsimile device etc. using an electronic photographic process. More particular, the invention relates to a charging roller, which has a better productivity due to its simple structure and having an excellent recycling ability.
2. Description of Related Art
In an electronic photographic type image forming device, a photoreceptor is charged by a charging device such as a charging roller, and then by exposing the photoreceptor's surface, an electrostatic latent image is obtained. After the electrostatic latent image is developed by the toner provided from a developing device to a toner image, the toner image is transferred onto a transfer paper. Then, an image is thus formed after a fixing process is performed.
In recent years, in view of environmental concerns, a charging device for a photoreceptor or a paper transfer device, which adopts a so-called contact charging type so that only a little amount of ozone is generated, has become a main product. Japanese Patent No. 3100625 discloses a charging material used in the above charging device. A multi-layered charging roller is of a general type with a structure such that a coating layer is coated on an intermediate layer with elasticity and conductivity surrounding a metal shaft of good conductor. When the contact charging process is executed on an electrified body such as the photoreceptor, if the straightness of the charging roller's edge line is not good, the contact state with the electrified body in the generating line direction of the charging roller becomes uneven. Because a poor electrification might occur, in general, the surface of the intermediate layer, which has elasticity and conductivity and is formed by rubber or foam, is ground to obtain a good straightness. This grinding operation is repeatedly performed in an order of a rough grinding, a medium grinding and a finishing grinding.
In addition, in the actual process, the metal shaft protruding form the two ends of the roller in the axial direction is kept rotatable. Mostly, the roller is rotated and a grinding stone is moved to press and contact with the roller, so as to grind the roller. Therefore, the slipping amount of the roller due to the grinding stone's press contact force is different at the roller's end and at the roller's central portion, and therefore, deviation between the grinding amounts at the roller's end and at the roller's central portion occurs easily. In addition, the grinding amount is smaller than the target value, and the grounded dimension becomes large, which causes a bounce phenomenon.
To prevent the aforementioned problem, the press contact forces respectively applied to the roller's end and central portion are changed, the grinding process is performed plural times, and each time only a little amount is grounded. Therefore, the manufacturing time is increased, and the device inevitably costs more. The roller's shape cannot be completely controlled, causing a drawback of the roller having an uneven shape.
In Japanese Laid Open No. H05-307279, when the above charging roller is used to charge in a non-contact manner, an air gap below 120 μm is required. Because the roller shape's unevenness is increased in the grinding process, it is very difficult to maintain the air gap below 120 μm across the longitudinal direction of the roller. In addition, even though the air gap can be maintained within 120 μm, charging unevenness might occur due to much gap unevenness. In order to reduce the charging unevenness, increasing the accuracy in the longitudinal direction of the roller is required, so that the grinding process has to be performed much more. Therefore, the manufacturing cost is increased.
In addition, in order to make the charging roller to be in contact with the electrified body firmly and to be driven by an accompanying rotation with the electrified body, in general, the charging roller uses a resilient unit such as a spring with a preset press pressure to press and contact with the electrified body. In this case, a press force is applied by a spring etc, to the two ends of the charging roller in the axial direction, and the charging roller's central portion in the axial direction is warped to a direction deviating from the electrified body. In order to avoid the above problem, the metal shaft (as the core metal of the charging roller) is thickened to increase the rigidity etc., but problems, such as high cost and heavy weight, may occur.
On the other hand, Japanese Laid Open No. 64-73365 discloses a charging roller with a structure such that a single-layered conductive resistant layer is covered surrounding a metal shaft of good conductor. However, similar to the aforementioned charging roller, a grinding process is also required. Therefore, the cost is high and the shape unevenness is great. Additionally, in view of contamination to the electrified body (the photoreceptor), a structure without the coating layer on the roller's surface is not practical. Furthermore, the conventional conductive unit has a rubber layer formed on the outer circumferential surface. When reusing the conductive base, removing the rubber layer is a time-consuming job, and therefore, the reproducing cost is increased.
Recently, from the environmental viewpoint, it has become desirable to make good products capable of being recycled. The part design capable of reuse becomes a portion of the material recycled. In consideration of recycling, the conventional conductive unit has a structure such that a low friction material is covered on the roller's surface. When the roller's surface is contaminated due to long time use, the contamination can be easily removed by washing and cleaning, so that reuse is possible. However, when there occurs physical defects such as pinholes or cracks, since the roller is fixed by adhesive, replacing the roller needs time and therefore the cost increases. In this case, the reuse is impossible and as a result, the conductive unit is wasted.